Method and apparatus for the production of composite sheet material and a sheet material produced thereby

ABSTRACT

A method and apparatus for the production of a composite sheet material and a material produced thereby are described, a typical product being a lightweight permanent or temporary formwork sheet of accurate thickness and parallelism made from material including glass fiber reinforced cement. A base or mould is moved through successive stations at which uncured but curable material is deposited in layers thereon. At least the final layer is consolidated by a local surface treatment with a vibratory screeder, the influence of which spreads through the material to promote intimate bonding together of the uncured layers. In a preferred form, the material comprises three layers, the first and third being of glass fiber reinforced cement and the central second layer being of cementitious material including a lightweight aggregate such as hollow microspheres. The screeder used on the top surface is accurately, guided with respect to the base or mould to provide an accurately controlled relationship between the faces of the finished sheet.

BACKGROUND OF THE INVENTION

This invention relates to a method and to an apparatus for theproduction of composite sheet material and to a sheet material producedthereby.

The invention has been devised especially in relation to sheet materialfor use in formwork for casting concrete, as an alternative orsubstitute for the conventional thick plywood sheets used at present.These plywood sheets have the disadvantages that their life isrelatively short, they are susceptible to swelling and splitting in thewet environment in which they are used, and the sheets need considerablesupport per unit area since wood is a relatively flexible material.

In view of these disadvantages of plywood sheets, it has been proposedto use sheets of a material comprising glass-fibre reinforced cement(grc). Such sheets have good release properties when used as temporaryformwork, are extremely stiff and strong and hence need less supportthan plywood for an equivalent permitted deflection, and are lesssusceptible to water damage than plywood. The sheets can be formed so asto provide a flat or alternatively a relief pattern surface on theconcrete being cast.

However, currently available grc sheets have certain disadvantages. Theyare much heavier than plywood of the same thickness. grc is relativelyexpensive because of the need to use an alkali resistant form of glassfibre in the mix to prevent attack by the cementitious matrix. Atpresent, grc sheets are laid down by a spraying process which isinherently likely to produce a slightly uneven top surface finish and sothat thickness of the sheets tends to vary and the surface finish isoften not even. The top surface of material has been rolled by hand toconsolidate the material and then floated to produce a more or less flatsurface but, in spite of the costly and time consuming nature of suchhand finishing processes, the resulting sheets are still slightlynon-uniform. When used in formwork, sheets are supported by woodenframeworks and it has often proved necessary to pack and shim theformwork extensively to achieve a satisfactory structure.

The invention is not limited to the manufacture of sheet material foruse in permanent or temporary formwork for concrete. It is alsoapplicable to the manufacture of sheet material for other uses, forexample as flooring or cladding or for other building purposes.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method andapparatus for the production of composite sheet material which are suchthat the thickness and surface finish of the material can be accuratelycontrolled and the material can be produced less expensively thanconventional grc sheets.

It is a further object of the invention to produce a new or improvedsheet material which is lighter in weight than conventional dense grcsheets.

A particular aim of the invention is to produce such a sheet materialsuitable for use in formwork, in which the above mentioned disadvantagesof currently available grc sheets are overcome or reduced.

According to a first aspect of the invention there is provided a methodof producing a composite sheet material comprising the steps of taking amould or base, depositing thereon a first layer of a curable material ina viscous flowable state, depositing on the first layer a second layerof a curable material in a viscous flowable state, the first layerhaving sufficient strength in its uncured state to support the secondlayer, consolidating the second layer by localised application ofvibration to the surface thereof, and allowing the materials to cure,the materials being such that on curing the layers bond together.

The expression "viscous flowable state" as used herein includes acondition in which the material is extremely viscous and can be causedto flow only under pressure or vibration.

The method may include the additional step of depositing one or morefurther layers of a curable material in a viscous flowable state on thesecond layer, the layer deposited before the or each such further layerhaving sufficient strength in the uncured state to support said furtherlayer, and consolidating the or each further layer by localisedapplication of vibration to the surface thereof.

In the performance of the method, the mould or base may movecontinuously through a plurality of successive stations at which thelayers are continuously deposited and consolidated.

The surface of the second or of a further layer may be consolidated bymeans which are guided and orientated accurately relative to the base ormould, so as to give accurate control of the relationship between thefaces of the sheet material produced.

In a preferred method, three layers are deposited, the curable materialof the first and third layers being glass fibre reinforced cement andthat of the central second layer being cementitious and including alightweight aggregate.

Irrespective of the number of layers, it is preferred that the curablematerial of each layer comprises a curable constituent and one or moreother constituents, all the layers having the same curable constituentto cause bonding together of the layers on curing.

The curable constituent may be hydraulic cement or alternatively may begypsum. The other constituents may include glass fibre or other fibrousreinforcements, lightweight aggregates such as hollow microspheres, airentraining agents, plasticisers, sand or fillers or any combination.

Viewed from a further aspect, the invention comprises an apparatus forproducing a composite sheet material comprising a support affording orsupporting a base or mould, first depositor means for depositing a firstlayer of curable material in a viscous flowable state on the base ormould, second depositor means for depositing a second layer of a curablematerial in a viscous flowable state on the first layer, and vibratorymeans adapted to apply localised vibration to the surface of the secondlayer in order to consolidate it.

The apparatus may additionally comprise one or more further depositormeans for depositing one or more further layers of curable material in aviscous flowable state on the second layer and further vibratory meansadapted to apply localised vibration to the surface of the or eachfurther layer in order to consolidate it.

The depositor means and the vibratory means may be disposed atsuccessive stations and the mould or base may be arranged to movesuccessively through the stations.

In this case, one or more of the depositor means may comprise a sprayhead adapted to deposit a glass fibre reinforced hydraulic curablematerial and the spray head may be movably mounted at the associatedstation to traverse across the direction of movement of the mould orbase.

The or each screeding means of the apparatus may be guided andorientated accurately relative to the base or mould, whereby therelationship between the surfaces of the composite sheet produced isaccurately controlled.

Viewed from a further aspect, the invention also comprises a compositesheet material when made by the method set out above.

The composite sheet material may comprise a matrix of cured hydrauliccement having three thickness zones defined by the presence of differentother constituents within the matrix of adjacent zones, the outer zonesincluding alkali resistant glass fibre as such another constituent andthe inner zone including a lightweight aggregate as such anotherconstituent, the opposite faces of the sheet material having anaccurately controlled relationship.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail by way of exampleonly with reference to the accompanying drawings in which:-

FIG. 1 is an end elevational view of an apparatus for producingcomposite sheet material;

FIG. 2 is a diagrammatic end elevational view of a mould or base used inthe production of the sheet material;

FIG. 3 diagrammatically illustrates the method of production of a sheetmaterial having three layers;

FIG. 4 is an enlarged diagrammatic view of a screeding and consolidatingstage in the process;

FIG. 5 is a cross-section through one form of sheet material embodyingthe invention and which may be produced by the process described withreference to FIGS. 2 to 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process to be described is carried out continuously orsemi-continuously using a base or mould having the cross-section shownin FIG. 2. This comprises a flat base plate 10 and a pair of sidemembers 11 which are of L-shape in the drawings but which could compriseupright plates. The side members 11 are adjustable for height relativeto the top surface of the base plate and have top guide edges 12 whichcontrol the positioning and orientation of a screeding bar to bedescribed.

FIG. 3 shows diagrammatically the method by which a three-layer sandwichsheet of material (illustrated in FIG. 5) is produced.

The apparatus includes first, second and third depositor stations,spaced apart successively along the direction of travel A of the mouldor base plate 10. At each successive depositor station, one layer of thecomposite sheet is laid.

In the example to be described, the first layer 16 which is deposited atthe first station 13 is a layer of glass fibre reinforced cement (grc).This is deposited in generally conventional manner by a spray headdisposed at the station 13, the spray head being arranged to traverseacross the base plate 10 from side to side as the base plate movescontinuously in the direction of the arrow A beneath the depositorstation 13. grc layer 16 is in a viscous but flowable condition. Inorder to provide a smooth surface and to ensure optimum compaction ofthe grc layer 16, a vibratory screeding bar 17 is used to screed the topsurface of the grc layer. It is preferred that the screeding bar 17 isguided and orientated accurately with respect to the base plate 10, forexample by being arranged to bear on the side members 11 of the baseplate 10.

While the grc layer 16 is still in an uncured state the base plate 10passes under the second depositor station where another layer ofmaterial is deposited.

In a preferred embodiment, this layer comprises a lightweight "mortar"which comprises a slurry of water and cement binding together alightweight aggregate. The lightweight mortar layer 18 is laid in aviscous flowable state as hereinbefore defined. Some form of vibratoryfeed may be provided at the second station 14 to ensure continuousfeeding of the layer 18 directly onto the uncured grc layer 16.

Shortly after the lightweight mortar layer 18 has been deposited at thesecond station 14, it is screeded by means of a second vibratoryscreeding bar 19 which again is guided so as to be, for example,accurately parallel with the undersurface defined by the base plate 10.The screeding bar 19 is preferably vibrated transversely of thedirection of movement of the mould, by means of a vibrator which isindicated at 20 in the enlarged view which forms FIG. 4 of the drawings.

Although the grc layer 16 has not cured when the lightweight mortarlayer 19 is laid on it, it must have sufficient strength in its uncuredstate to support the second layer. After the second depositor station,when the base plate 10 passes under the second screeding bar 19, somedifficulties might be expected because of the relatively flowablecondition of the underlying grc layer. One might expect that the grclayer 16 and the lightweight mortar layer 18 would intermingle to anunacceptable extent under the influence of the vibratory screeding bar19 or that the relatively flowable state of the layer 16 would preventthe layer 18 from being acceptably consolidated and levelled.

Surprisingly, we have found that the screeding can be performed whilstboth the lightweight mortar layer 18 and the grc layer 16 are in anuncured, flowable and wet condition, with considerable advantages.

These advantages can be enhanced by the use of the type of mould shownin FIG. 2 and described above. At the second depositor station, the sidemembers 11 are raised to the dotted line position shown at 21, forexample by means of suitable cams provided on the conveyor apparatus.The side members can then constrain the lightweight mortar and the grcto a region above the base plate 10 and prevent material being lost overthe sides of the apparatus. In addition to preventing wastage, thisenables a state of fluidisation to be built up in the grc substratelayer 16 when the two layers of the material pass under the screedingbar 19.

To explain this phenomenon in more detail, reference should be made toFIG. 4 of the drawings which illustrates diagrammatically the region ofthe screeding bar 19.

The mould base plate 10 is travelling in the direction of the arrow A asbefore and carries the grc layer 16 which, it will be recalled, is in anuncured condition of sufficient strength to support the second layer 18.The two layers pass under the screeding bar 19 which is vibrated in adirection transversely of the direction of motion A. The material isconstrained to the base plate 10 by means of the elevated side membersin the position 21 and, in the region immediately beneath the screedingbar, the vibration of the screeding bar fluidises the cementitiousbinder of the material beneath it.

The effect of the vibration is felt primarily immediately adjacent theunderside of the screeding bar 19 but there is also a radially extendingzone of influence which is indicated generally at 22 in the drawings bya series of part circular arcs. This zone of influence is of coursepart-cylindrical, extending right across the base plate 10. A certainamount of fluidisation takes place within the zone of influence.

This fluidisation causes the consolidation of the lightweight mortar 18and its intimate bonding to the wet substrate layer 16 of grc.

The effect is confined to the zone of influence 22. In the regions ofthe layer 16 immediately before and after the zone of influence 22, thegrc layer 16 retains its uncured strength to which reference haspreviously been made. The fluidised material of the layer 16 within thezone of influence 22 is therefore confined between the less stronglyfluidised adjacent regions and, in spite of its relatively liquid state,cannot escape from the zone 22 because it is so constrained.Additionally, the material is constrained at the ends of the screedingbar 19 by the elevated side members 11.

The upper layer 18 is fluidised and consolidated by the action of thevibrating screeding bar 19. The vibrations are transmitted through theupper layer 18 of the lower layer 16, creating in it a degree offluidisation. Thus, a very good bond is obtained between the grc layerand the lightweight mortar layer so that the cement matrix iseffectively continuous across the junction and only the otherconstituents differ. The upper surface is flat and smooth and, if theposition and orientation of the screeding bar 19 is controlled by theside edges 12, the surface can be exactly parallel to the under surfaceof the sheet defined by the base plate 10, or otherwise have anaccurately defined relationship with the undersurface.

Although the full advantages of the method may be achieved using theside members 11 to constrain the material at its edges, a satisfactoryeffect may be obtainable without the side members (except that the edgesof the finished sheet need to be trimmed).

The first two layers of the material have thus been laid and thematerial passes under the third station 15, where a further grc layer 23is added, again using a transversely moved spray head similar to thatprovided at the station 13. The further grc layer 23 is screeded bymeans of a further screeder bar 24 which operates in a precisely similarmanner to the screeder bar 19. It will be appreciated that the zone ofinfluence of the screeder bar 24 may or may not extend through to thelowermost layer 16 but that it will primarily promote intimate bondingof the layers 18 and 23 of lightweight mortar and grc respectively.

The three-layer sheet material emerging from the zone of influence ofthe screeding bar 24 continues along the conveyor system and is allowedto cure. An advantage of the process outlined is that, since thematerials are wet when the three layers are superposed and since thecement matrix extends throughout the sheet material, all the layers curetogether and hence there is no problem of different curing stagescausing separation of the layers or curling of the sheet material.

The process described above is for the manufacture of a three-layersheet material of sandwich construction, faced with grc. By usingdifferent materials, other types of sheet can be produced, for example,a sheet in which a substrate of lightweight mortar has a single facingsheet of grc. The method can clearly be extended to provide other typesof sandwich or multiple sandwich material. It may or may not benecessary to provide vibratory screeding at each stage of the process.It will also be appreciated that the vibratory screeding bar 17 could beomitted from the apparatus shown although this does enable the firstlayer to be laid in a viscous condition and to be thoroughlyconsolidated.

FIG. 5 shows part of a composite sheet of material which is produced bythe method, operation of which was described with reference to FIGS. 2to 4 of the drawings. This sheet material is only one example of variousmaterials which can be produced using the method and has been developedspecifically for use as a formwork for the casting of concrete. It maybe used either as a temporary formwork, which is removed leaving theconcrete surface exposed, or as a permanent formwork which is left inplace after the concrete has set.

The material shown includes three layers 16, 18, 23. The central corelayer 18 which occupies about two thirds of the thickness of the sheetis made of a lightweight mortar and this core is faced on each of itsopposed faces by a layer of glass fibre reinforced cement having athickness about one sixth that of the sheet. Each grc facing layer ismade by spraying and consolidating a mixture of chopped alkali-resistantglass fibre of the type marketed under the Registered Trade Mark CEMFIL,together with a slurry of Portland cement, fine aggregate such as sandand water. By way of example, the ratio of cement, sand and water in theslurry may be 30:15:9 and we have found that the desirable quantity ofglass fibre to be introduced into this slurry is approximately 5% byweight of the slurry. Proportions may vary according to the type ofcement or fine aggregate used, or other factors. Alternative fineaggregates are crushed limestone or PFA (pulverised fuel ash).

The core is formed from a lightweight mortar which includes 100 partscement, between 50 and 200 parts water and between 50 and 300 parts ofPFA CENOSPHERES by weight. PFA CENOSPHERES are an example of alightweight aggregate and comprise small hollow gas-containingmicrospheres which add to the volume of the cement without addingsubstantially to its weight. When cured, the cement matrix effectivelyforms a skeleton structure surrounding the bubbles.

We have found that a useful composition is one part by weight of cementto one part by weight of CENOSPHERES with between 0.65 and 1.1 parts byweight, (say 0.75 parts by weight) of water. However, the proportion ofCENOSPHERES to be added can be varied between one half the weight ofcement to three times the weight of cement. At the lower end of thisrange, both the strength and the density of the composition are quitehigh. At the upper end of the range, the density is extremely lowcompared with solid cement but the strength is also somewhat reduced.

Other forms of microspheres can be used instead of the cenospheres, orfoamed plastics beads may be substituted. It may be desirable to producea fire resistant material, in which case the use of organic materialsmay be objectionable and an inorganic lightweight aggregate may bepreferred.

To improve the strength, some fibrous reinforcement may be added, forexample alkali-resistant glass fibre or various types of syntheticplastics fibres.

The quantity of water to be used may vary considerably with the quantityof lightweight aggregate because, for example, PFA CENOSPHERES have avery high surface to weight-ratio and tend to adsorb a considerablequantity of water onto their surfaces, reducing the amount of free waterwhich can be used for hydrating the cement.

In addition to the CENOSPHERES, air may be incorporated in the mix bythe use of an air entraining additive. In addition, or alternatively, aplasticiser may be added. This enables the water content to be reduced.The action of some plasticisers is also time related and, for example, avery viscous mix incorporating a plasticiser may be quite flowable for acertain length of time but will undergo a pseudo-setting process after atime delay. This effect may be used with advantage in making thecomposite sheet.

The composite sheet material described, particularly when vibratoryscreeding is used in its manufacture, forms a cohesive sheet with hardand rigid facings which may have any desired surface finish, the core ofthe material being light in weight and the entire material having acontinuous cementitious matrix, only the aggregate or reinforcementdiffering between the layers. The thickness of the sheet and parallelismof the faces can also be controlled by the means described above.

Such a material is extremely useful for formwork as a substitute oralternative for plywood. It is provided with strength and rigidity byits grc facings and the weight is reduced by the lightweight mortarsubstrate or core, compared with conventionally available solid grc.

In the example described, there are no large air voids in the materialso that it is of homogeneous composition and strength throughout. Thisis achieved by use of the vibratory screeding method, combined with theuse of CENOSPHERES to provide the lightweight core. Where air entrainingagents are used, it has been found that the vibratory screeding does notprevent the entrainment of air.

Referring more specifically to the apparatus shown in FIG. 1 of thedrawings, the base plate 10 and side members 11 have already beenreferred to. These are mounted on a carriage 25 which is provided with aplurality of wheels 26 running on tracks 27. The tracks run beneath aplurality of stations, of which the station 13 is illustrated. At thisstation, there is an overhead transverse rail 28 carrying the spray head29 previously referred to for depositing the grc material on the baseplate 10. The spray head traverses from side to side across thedirection of movement of the carriage 25, along the overhead transverserail 28. Immediately beyond the position at which the spray is laiddown, the material forming the first layer 16 is screeded by thescreeder bar 17 which is fitted with a vibrator 30. The screeder bar 17is loosely mounted in slotted brackets 31 and is arranged to ride on theside members 11 of the base plate to control the thickness andparallelism of the layer 16 as described above. It will be noted thatthis takes place irrespective of any slight rocking or irregularmovement of the carriage 25 on the tracks 27. The remaining stations ofthe apparatus are similar to the station 13 described above, with theexception that the spray head 29 may be replaced by depositor means forother types of material which do not require to be sprayed, for examplethe lightweight cementitous mortar referred to above.

It will be appreciated that many modifications may be made to theapparatus and to the method and composition described above, withoutdeparting from the scope of the invention. Such modifications, toproduce different types of material for different purposes will bereadily apparent to those skilled in the art, on the basis of theforegoing description.

An alternative to using chopped glass fibre to reinforce the facinglayer(s) is the use of glass fibre mat of a woven or non-woven type. Inthis case a thin layer of viscous slurry is laid, the mat is unrolledonto this layer and a further slurry may then be added. It is believedthat, where such a facing is laid as an initial layer, it may besufficient to add a core layer and then consolidate both simultaneouslyby vibratory screeding.

Of course, fibres other than glass, for example metal or plasticsfibres, might alternatively or additionally be used in any of thelayers.

If desired the lateral constraint can be supplied by edge strips whichremain part of the finished sheet and which are only temporarily securedto the mould.

The invention is likely to prove particularly useful in adding a layerto a core or substrate of cementitious material which is not reinforcedwith fibres, because, in an uncured condition, such materials tend to bemore flowable than fibre reinforced materials. The invention enables afibre reinforced material to be deposited and consolidated on thesurface of an unreinforced material without the need for preliminarycuring of the unreinforced material and its consequent disadvantages.

I claim:
 1. A method of making a composite product including distinctlayers of two different water-based curable compositions capable ofbonding together on curing, the method comprising the stepsof:depositing a first layer of a first of said curable compositions ontoa base member while said first composition is wet and uncured and in aviscous state having adequate strength to support a second layer butcapable of flowing under the influence of vibration; depositing directlyand immediately onto said wet first layer a wet second layer of thesecond of said curable compositions without any prior treatment of thewet first layer other than levelling, the second composition being wetand uncured and in a viscous state but capable of flowing under theinfluence of vibration and the two layers meeting at an interface;temporarily creating a localised zone of fluidization extending throughsaid wet second layer to the interface between the wet first and secondlayers by the application of vibration to a small localised area of thesurface of the wet second layer to promote intimacy and subsequentbonding of the first and second layers, the first and secondcompositions remaining viscous outside the said localised zone offluidisation; and allowing the first and second compositions to curetogether to form the composite product.
 2. A method according to claim 1wherein at least one further wet layer is deposited on the wet secondlayer, such further layer being of a water-based curable compositioncapable of bonding to the composition of the preceding layer and all ofsaid compositions being in a wet, uncured and viscous state capable offlowing under vibration, the successive wet layers being depositeddirectly and immediately one on another to define interfaces between thelayers, and a temporary localised zone of fluidization being created,extending through at least the topmost layer to its interface with thepreceding layer by the application of vibration to a small localisedarea of the surface of the topmost layer to promote intimacy andsubsequent bonding of at least the topmost and preceding layers, thecompositions remaining viscous and retaining their strength in theuncured state outside said localised zone of fluidization, thecompositions finally being allowed to cure together to form thecomposite product.
 3. A method according to claim 1 or claim 2 performedas a continuous process in which said base member moves through aplurality of stations at which said compositions are continuouslydeposited and said localised vibrations are applied.
 4. A methodaccording to claim 3 wherein side walls are provided on said base memberand serve to guide and orientate means for applying said localisedvibrations as the base member passes through at least one of saidstations so as accurately to control the thickness and surface contourof the product.
 5. A method according to claim 2 wherein first, secondand third layers of water-based, curable compositions are depositedsuccessively, the first and third layers being of the same curablecomposition and the second layer being of a composition different fromthat of the first and third layers.
 6. A method according to claim 5wherein the composition of the first and third layers is glass-fiberreinforced cement and that of the second layer is cementitious andincludes a lightweight aggregate.
 7. A method according to claim 1wherein the water-based curable composition of each layer comprises acurable constituent and at least one other constituent, all of saidlayers having the same curable constituent to cause bonding together ofthe layers on curing.
 8. A method according to claim 7 wherein thecurable constituent is hydraulic cement.
 9. A method according to claim7 wherein the curable constituent is gypsum.
 10. A method according toclaim 7 wherein at least one of said layers includes fibers as suchother constituent.
 11. A method according to claim 10 wherein the fibersare of glass.
 12. A method according to claim 9 wherein at least one ofsaid layers includes a lightweight aggregate as such other constituent.13. A method according to claim 12 wherein at least one of said layersadditionally includes a fibrous reinforcement as such other constituent.14. A method according to claim 12 wherein at least one of said layersadditionally includes an air entraining agent as such other constituent.15. A method according to claim 12 wherein at least one of said layersincludes a plasticiser as such other constituent.